The present invention is directed to a method for friction welding of a first part having a smooth-faced functional outside portion to a second part, comprising the steps of retaining the first and the second parts axially opposite one another in a torsion-proof manner in respective separate holding devices, of which at least one of the holding devices is arranged to be rotatable, heating the parts to a pasty state by frictional heating, rotating the parts relative to one another, pressing one of the parts into the other of the parts with a decreasing relative rotation and with increased axial force to effect friction welding, embossing, in the region of the holding face of the part in the holding device, regular positive-connection faces which match with corresponding positive-connection faces which match with corresponding positive-connection counterfaces in the holding device such that the first part is retained in the holding device in a positive torsion-proof manner, and thereafter stopping relative rotation of the parts.
The present invention also relates to a holding device in a friction-welding machine for the positive, torsion-proof holding of a first part for friction welding to a second part. The first part has, on a side facing the holding device, at least one at least approximately cylindrical face and an at least approximately axis-perpendicular face, along which the first part alignable in the holding device and retained in a tilt-proof manner and safe against falling out. The cylindrical face and the axis-perpendicular face of the first part merge into one another along a circularly extending workpiece edge. Positive-connection faces on the first part and matching negative positive-connection counterfaces on the holding device for the positive torsion-proof-retention of the first part in the holding device, at least in a region of the positive-connection faces, are made harder than the positive-connection faces of the first part.
In particular, the present invention is directed to a friction welding method and apparatus as is carried out or is in use in the industrial mass production of, for example, gear parts.
DE 3,802,300 C1 discloses the friction welding of a thin-walled tubular part to a solid part, but does not deal with the method of fixing the parts during friction welding. The problem addressed in that document is the high hardness of the tubular part and a frictional heating and sufficient softening of this same part.
Friction welding affords important advantages in relation to other welding methods, because it is highly efficient, makes no demands on the pretreatment of the faces to be joined, offers the possibility of welding through even large cross-sections over the entire area and works virtually free of distortion, so that finished parts can also consequently be joined without the need for reworking. Friction welding is therefore expedient for many instances of use with a rotationally symmetrical weld point, not only in mass production, but also in production with small batch sizes.
During friction welding, however, very considerable torques occur in the fixture of the workpieces to be welded together, above all towards the end of the welding operation. The parts therefore have to be held in the corresponding holding device with high torsion-proofing. Parts with regular positive-connection faces present in any case on account of their function afford some advantages in this respect because, by utilizing these positive-connection faces, such parts can be held in the appropriately adapted holding device positively, that is with very high torsion-proofing. The toothing of gearwheels, the blade ring of turbine wheels or pump impellers, the ring of holes of fastening flanges, spline profiling are examples of the many parts with such faces.
For a non-positive gripping in the clamping tool, parts which are rotationally symmetrical and smooth-faced on the outside must have high stability, that is large wall thicknesses, and also large engagement faces for correspondingly large and stable clamping jaws, so that the parts can be retained non-positively with sufficiently high torsion-proofing, because extremely high radial clamping forces have to be exerted on the workpiece for this purpose in view of the low coefficient of friction between metal parts.
These conditions are often absent at least on one of the parts to be welded together in the friction-welding method, either because the part is too thin-walled or it is too small in diameter on the circumferential face available for clamping or has too short an axial dimension. This occurs mostly in the case of essentially disc-shaped parts. So that the welding method favorable in production terms can nevertheless be employed in such instances, suitable positive-connection faces which, apart from the transmission of torque during friction welding, have no further function, have been worked, "artificially", as it were onto such workpieces on the circumferential face provided for holding the workpiece in the clamping tool. Examples of parts of this are, in addition to the essentially disc-shaped parts already mentioned, also pot-shaped parts of small diameter, e.g., screws, or thin-walled cylindrical, tubular or disk shaped parts. These parts offer no engagement faces for the vigorous application of clamping jaws.
An object of the present invention is to simplify substantially a method and holding device specifically to the effect that a positive connection required only for the transmission of torque during friction welding can be substantially simplified.
According to the present invention, this object has been achieved wherein the positive-connection faces located in the first part are produced only in the holding device by embossing carried out by the positive-connection counterfaces formed in the respective holding device and made cutter-like. The cutter-like positive-connection counterfaces are arranged at an acute angle (.alpha.) of less than 45.degree. to an axial force direction, as seen in an axial section, thereby digging into a peripheral workpiece edge of one part in a force-transmitting manner. The embossing step is carried out either directly before the friction welding by a separate axial pressing force which is applied separately or during the friction welding by utilizing the axial pressing force thereof. Because the positive-connection faces on the parts are produced automatically an operation akin to embossing during the friction welding, the outlay in terms of time and machinery for a separate production of positive-connection faces of this type is avoided. Also on account to the very small, but many, positive-connection faces in this region of the workpiece, the parts can also be kept smaller and lighter in comparison with larger positive-connection faces produced separately. The parts can therefore be produced more quickly and more cost-effectively and, furthermore, for reasons of weight, afford particular functional advantages.
Further advantages of the embodiment according to the invention of the holding device include a small constructional space of the holding device, thus also making friction welding possible in confined conditions of space, e.g., on the bottom of a pot-shaped part; a simple configuration of the workpiece-holding device without movable parts; nevertheless, a secure and positionally accurate clamping of the first part to be held; and also axially very short and thin-walled parts can be clamped reliably and therefore friction-welded, which has hitherto been impossible with a small constructional space.